Rheostat



Sept 4, l945- w. F. PENRsE v 2,383,987

- RHEo-STAT 'Filed oct. so, 1943 I Wfl/fam EP02/05e ATTORNEYS PatentedSept. 4, 1945 UNITED STATES PATENT oFFlcE William F. Penrose,

Empire Electric Brake tion of Delaware Irvington, N. J., assigner toCorporation, a corpora.-l

Appucmon october 8o, 194s, serm No. 508,292

' 7 Claims.

'This invention relates to rheostats and has for :itsobject certainimprovements in the construcf element extend out of contact with thebase freely exposed for the dissipation of heat. A sliding contact ismovably secured to the base with the arm end ofthe resistance element,and two spaced holes 21 and, extending to a recess 29, for securingterminal l2,

`The back part of the b ase is also provided with a cylindrical hollowinterior 20, dened by the inner annular wall 23 and the terminal portion2l. The terminal portion is also provided with a recessed slot 3| toreceive the terminal i4. 'I'he inner annular wall 23 terminates in'fanannular shoulder 32 against which the,continuous contact ring I3 mayrest. A round hole Il extends throush thereof in sliding engagement withthe exposed ends of the zig-zag resistance element bends. These andother features of the invention 'may be better understood by referringto the accompanying drawing in conjunction with the i'ollowingdescription, in which:

Fig. 1 is a plan view of'a rheostat illustrative l of the invention;

Fig. 2 is a. section on the line 2-2 of Fig. 1; Fig. 3 is a front view,with the turning knob shown in dotted lines; Fig. 4 is a section on theline 4-4 of Fig. l; Fig. 5 is a detail of the sliding contact assemblyof Figs. 1, 2 and 3; yand Y Fig. 6 is a cross-section on the line H oiFig. 5. l

The rheostat shownvcomprises-an insulating base I0, a resistance elementii, one end terminating in a terminal l2, a continuous contact ring i3separated from the resistance element, one end terminating in a terminali4, and a biiurcated sliding contact or brush i5 mounted upon anassembly i6.

Referring to the construction of the insulating base, it will be seenthat it is generally circular in shape and is advantageously made ofporcelain. The front part of the base consists of an enlarged,

' and for the most part solid, cylindrical portion 2li having .an outerannular wall 2| separated from the interior of the base so. as to forman annular the center of the solid ortion of the base, the back entranceto the -ho e having a recess Il of predetermined size and ape. A stop orboss 3i, generally V-shaped s that its sides 35' and 35" are insubstantial al ent with the center of the hole 3l, is formed by a raisedintegral portion of the base, extending from the bottom of the hollowinterior 30 adjacent the terminal portion .'il. The reverse, or frontIside of the base con-- tains an enlarged circular recess It at theentrance to round hole 33, and a recessed direction slot 3l terminatingwith the recess 2l.

As shown more particularly in Fig. 4, the resistance element i l isfolded back and forth upon itself, in a zig-zag formation, with orwithout metal to metal contact depending upon the type of resistanceemployed; 'I'he element is advantageously a relatively thin flatmetallic ribbon which can be bent in this manner to any suitable length,the element as a whole being relatively straight when first formed butadapted to be bent into a circular generally cylindrical shape. In apresently preferred construction, the resistance element is made ofnickel-chromium alloy, and its exterior surface is provided withanon-conducting oxide coating so that if bends of the element shouldcontact one another, a short circuit is avoided. 'I'he cross-sectionalarea and the length of the element are predetermined to provide thedesired amount of ohmic resistance. Its lateral end 40 is passed througha'hole 4I in the terminal l2 and appropriately secured therein, as bycompressing the end of the element in the hole, to make permanentcontact with the terminal. 'I'he terminal, -with the resistance elementsecured thereto, is placed in position in the base, as shown in Fig. 1,thelateral end 40 resting in the recess 26. A pair of rivets 42 and Ilis passed throuh the holes 2l and 2l, respectively, and the free ends ofthe rivets are turned over to form heads M and l5, respectively, asshown in Fig. 4, to secure the terminal to the back of the base.Suitable plastic porcelain cement is placed in the anular recess 22.'I'he resistance element i i is then bent around the outside of theinner annular wall 23, and the lower portion of the element is insertedin the annular recess 22, the free end of the resistance element beingbrought to the terminal portion 24 of the base, as shown. 'I'his servesto embed the resistance element in the cement and to hold the samepermanently in position in the recess after the cement hardens.

The terminal I4 is an integral part of the continuous circular contactring I3, the free end of which does not contact with the remainder ofthe ring or its terminal. To mount this unitary structure, plasticporcelain cement 46 is first smeared on the inside face of the innerannular wall 23, the terminal I4 is inserted in the slot 3l and thebottom of the contact ring is pushed toward the annular shoulder 32.Additional porcelain cement 4' is then placed in the circumferentialrecess 41 formed by the ring and the beveled portion 25 of the innerannular wall. The ring is provided with a plurality of holes 48 whichare filled with the cement 4B' in order to hold the ring in position.

'I'he sliding contact assembly I6 is shown in detail in Figs. 2, 5 and6. It comprises a shaft 50, one end of which is flattened a suitablelength to provide a pair of opposite flat faces 5I and 52, and shoulders53 and 5l, adapted to receive in succession: a metal washer 55 with ahole to fit snugly over the flattened end of the shaft so that therwasher may rest against the shoulders, a larger insulation washer 58,the bifurcated sliding contact I5, a similar insulating washer 5l, and asimilar metal washer 58. These elements are held against one another byhammering the end of the shaft to form a head 59. The bifurcated slidingcontact is provided with a shaft hole 60 substantially larger than theshaft so that it may be kept out of contact with the shaft itself. Tothis end, the sliding contact is provided with a pair of integralopposite ears 6I and 62, located between juxtaposed slots 53 and 64 ineach insulation washer; and the ears are bent into the slots of thefirst insulation washer 58, thereby preventing the sliding contact frommoving latorally against the shaft. The sliding contact is bifurcated,as shown, to provide an upper arm 6I and a lower arm B8, and isadvantageously made of sheet copper which is suiiiciently flexible topermit bendingA of the arms to make certain that the upper arm may slidein contact with the exposed ends i1 of the zig-zag resistance element IIand the lower arm in contact with the exposed rim il of the contact ringI3, when the rheostatis completely assembled.

A stop pin 10 fits in the shaft a suitable distance from its end. Thepin is in alignment preferably with the sliding contact and extendscompletely through the shaft so that the ends of the pin are freelyspaced from a bushing II rotatably mounted on the shaft The end of thepin in alignment with the sliding contact is adapted to be brought inengagement with sides and 35"' of stop 38. I'he end of the bushingadjacent the stop pin is provided with an hexagonal head I2 adapted tofit snugly in the recess 34 of the base. The other end of the bushing isthreaded to receive an hexagonal nut 13. A split washer 14 fits in agroove 15 of the shaft to hold the shaft in-place. The other end of theshaft contains a fiat face 18 to receive a screw ll in a turn knob 13attachable to the shaft. The knob has an arrow Il to be kept inalignment with the sliding contact to indicate the position of its armson the zig-zag resistance element and the contact ring.

The sliding contact assembly is initially assembled in the form shown inFig. 5. The bushing 1I is then mounted on the free end of the shaft sothat its hexagonal head l2 is in contact with the stop pin 10, afterwhich the split washer Il is fitted in the groove 15. of the shaft tohold the bushing in place. The -free end of the shaft and bushing areIthen inserted into the round hole 33 and are turned until the hexagonalhead is fitted into the recess 34. The hexagonal nut 'I3 is then screwedonto the threaded end of the bushing until the nut is in engagement withthe front of the base in the circular recess 3l. The knob 'I8 is thenmounted on the free end of the shaft, its screw 'II being brought tobear on the flat face I6 when the arrow is in substantial alignment withthe sliding contact. Movement of the turning knob to the right or leftcauses a corresponding movement of the arms and 66 on the exposed ends61 of the zig-zag resistance element I I and on the exposed rim E3 ofthe continuous contact ring Il, respectively.

An advantage of 'the ribbon or tape form of zig-zag resistance elementis that it offers a maximum amount of heat-radiating surface for a givencross-sectional area. The size of the ribbon ortape and the size of theannular recessed channel in the base may be so coordinated as to providean exact amount of ohmic resistance uniformly distributed over the spaceallotted to each step of the resistance. It is advantageous to securethe zig-zag resistance element and continuous contact ring to the basewith a cement of substantially the same expansion and contractioncharacteristics as the base. A porcelain base and, hence, porcelaincement are suitable for the purpose. This greatly reduces the tendencyof the cement to crack away from the base, and the resistance elementand contact ring are, therefore, securely held in place.

It will be clear to those skilled in this art that the device ,lustdescribed is merely an example of the invention and that modificationsare possible and feasible. The construction disclosed greatlyfacilitates and expedites the dissipation of heat from the zig-zagresistance element. Placing the bends and contiguous portions of oneside of the zig-zag resistance element in the recessed channel andsecurely embedding the same in that position by filling the spacesbetween and around the element serves to leave it mounted in the base inan unusually strong manner. This l construction virtually makes theresistance element an integral part of the insulating base itself.

The continuous stationary contact is mounted securely in the base in themanner shown so that it, like the resistance element, can withstandviolent shaking and shocks without becoming loosened or separated fromthe base proper. Any other arrangement of contact may of course beemployed including a flexible connection.

The rheostat of the invention lends itself to ready manufacture becauseof the simple manner in which the several parte may be prepared prior totheir assembly.

I claim: f

1. In a rheostat, the improvement comprising an insulating base, arecessed channel in said base, two lead-in terminals, a zig-zagpreformed resistance element connecting one of said terminais,` acontinuous stationary contact securely mounted in said base andconnecting the other of said terminals, the bends and contiguousportions of one side of said zig-zag preformed resistance element beingsecured in said recessed channel, the bends and contiguous portions ofthe other side of said zig-zag resistance element being out of contactwith the base and freely exposed for the'dissipation of heat, a slidingcontact having one arm in sliding engagement with the exposed ends ofthe zig-zag resistance element bends and another arm in slidingengagement with the continuous stationary contact, and means associatedwith the base for moving said arms simultaneouslyback and' forth oversaid zig-zag resistance element and continuous stationary contact.

2. A rheostat according to claim 1, in which the recessed channel issubstantially annular.v

and the sliding contact is secured to a rotatable shaft mounted in thebase.

3. In a rheostat the combination of a base of insulating material havinga recessed channel therein, with a zig-zag preformed resistance elementhaving generally parallel contiguous portions ioining bends at oppositesides thereof secured in said channel with one side thereof includingbends and a substantial amount of said contiguous portions extending outof contact with said base and freely exposed for the dissipation of heatgenerated in said element, and a moving contact supported by said basein sliding engagement with an exposed portion of the said element.

4. In a rheostat the combination of a substantially circular cylindricalbase of insulating maelement, and a moving contsct'arm rotatablysupported by said base in sliding engagement with an exposed portion ofsaid element.

5. A rheostat according to claim 3. in which a stop formed by a raisedportion of the base is provided in the hollow interior adjacent to theterminal portion adapted to stop movement of the sliding contact. l

6. In a rheostat, the improvement comprising an insulating base. thefront part of the base forming an enlarged essentially solid portionhaving an outer wall separated from the interior of the base to form anannular recessed channel, the back part of the base having an inner wailadiacent the recessed channel and extending beyond the outer wall, saidinner and outer walls terminating in an essentially solid terminalportion, a hollow interior dened by said terminal portion and innerwall, two lead-in terminals secured to-the terminal por# tion, alzig-zag preformed resistanceelement connecting one of said terminals,the bends and contigu'ous portions of one side of the zig-zag preformedresistance element being secured in said annular recessed channel, thebends and contiauous portions of the other side of the zig-sagresistance element being out of contact with the base and freely exposedfor the dissipation'of heat, a continuous stationary contact securelymounted in the hollow interior of the base and connecting the other ofsaid terminals, and a sliding contact having one arm in slidingengagement with the exposed ends of the zig-ng resistance element bendsandanother arm in sliding engagement with the continuous stationarycontact, said sliding contact being secured to one end of a' rotatableshaft mounted ln the base. '1. A rheostat according to the precedingclaim, in which a bushing is rotatably mounted on the shaft, the bushingbeing stationarily secured t0 the base, and means at the other end ofthe shaft for rotating the same within the bushing.

WILLIAM Il'. 0 =1

